Conventionally, in the photolithography technique, an exposure device for transferring a fine circuit pattern onto a wafer, and thereby producing an integrated circuit is widely utilized. With higher integration and higher functionalization of integrated circuits, the microsizing of the integrated circuit is advancing. The exposure device is hence required to form a circuit pattern with high resolution on a wafer in a deep focal depth, and shortening of the wavelength of the exposure light source is being advanced. The exposure light source is further advancing from conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm) begins to be employed.
Furthermore, to respond to the next generation integrated circuit in which a line width of a circuit pattern is 70 nm or less, an immersion exposure technique and a double exposure technique, using an ArF excimer laser are considered to be effective. However, those techniques are estimated to merely cover up to the generation that the line width is 45 nm.
Under the foregoing technical trends, a lithography technique typically using, as an exposure light source, light having a wavelength of 13 nm among EUV lights (extreme ultraviolet light) is considered to be applicable over generations that a line width of a circuit pattern is 32 nm, and is attracting attention. The principle of image formation in the EUV lithography (hereinafter abbreviated as “EUVL”) is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The optical system member of an exposure device for EUVL is such as a photomask and a mirror, and is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate, and (3) an absorber layer formed on the reflective multilayer. For the reflective multilayer, it is investigated to form an Mo/Si reflective multilayer in which an Mo layer and an Si layer are alternately laminated, and for the absorber layer, Ta and Cr are investigated as a forming material. For the substrate, a material having a low coefficient of thermal expansion is required so as not to generate a strain even under irradiation with EUV light, and a glass and the like having a low coefficient of thermal expansion is investigated.
A fluorine-containing synthetic quartz glass is proposed as a material having high initial transmission in respect to high power vacuum ultraviolet ray and having high durability (see Patent Document 1).
Furthermore, a TiO2—SiO2 glass which is a TiO2-containing synthetic quartz glass is known as an extremely low thermal expansion material having a coefficient of thermal expansion (CTE) smaller than that of a quartz glass. Additionally, because the coefficient of thermal expansion can be controlled by TiO2 content in the glass, a zero expansion glass having a coefficient of thermal expansion close to zero is obtained. Therefore, the TiO2—SiO2 glass has the possibility as a material used in an optical system member of an exposure device for EUVL.
Patent Document 2 proposes a TiO2-containing silica glass having a fictive temperature of 1,200° C. or lower and a fluorine concentration of 100 ppm or more as a material extremely suitable for a material of a member constituting an optical system used in EUVL where the glass is a fluorine-containing TiO2—SiO2 glass in which a coefficient of thermal expansion at 0 to 100° C. is 0±200 ppb/° C., and changes in a coefficient of thermal expansion with temperature is small, that is, a temperature range in which a coefficient of thermal expansion is nearly zero is broad, and in which homogeneity of a coefficient of thermal expansion and mechanical properties in the glass is excellent.
Production processes of a fluorine-containing synthetic quartz glass and a fluorine-containing TiO2—SiO2 glass include the following several methods.
(1) One method is that a porous glass body is obtained, in a soot process, by depositing and growing quartz glass fine particles (soot) that is obtained by flame hydrolyzing glass forming materials. There is a production processes of obtaining a fluorine-containing glass body by treating the obtained porous glass body in a fluorine-containing atmosphere to introduce the fluorine into the porous glass body, and thereafter, heating the porous glass body to a transparent vitrification temperature or higher, thereby transparent-vitrifying the porous glass body. The soot process includes an MCVD process, an OVD process and a VAD process, depending on the preparation manner. In the case of producing a fluorine-containing TiO2—SiO2 glass, TiO2—SiO2 glass fine particles (soot) obtained by flame hydrolyzing or heat decomposing an Si precursor and a Ti precursor each serving as a glass forming raw material are deposited and grown, thereby obtaining a porous TiO2—SiO2 glass body.(2) As the soot process, there are production processes, in which fluorine-containing materials are used as glass forming raw materials or the glass forming materials are subjected to flame hydrolysis or thermal decomposition in a fluorine-containing atmosphere to obtain a fluorine-containing porous glass body, and thereafter, a fluorine-containing TiO2—SiO2 is obtained.(3) There is a production process for obtaining a fluorine-containing TiO2—SiO2 glass body, in a direct process, by using fluorine-containing materials as glass forming raw materials, or hydrolyzing or oxidizing glass forming raw materials in an oxyhydrogen flame of from 1,800 to 2,000° C. in a fluorine-containing atmosphere.
Of the above production processes, the process that the producing is easy and fluorine can relatively uniformly be introduced is the process (1). However, even in the process (1), there are the following problems: the temperature during treating a porous glass body in a fluorine-containing atmosphere is required to be high temperature of 400° C. or higher in order to introduce 1,000 ppm or more of fluorine and therefore O ring is required to be cooled with water in order to secure gas tightness of a furnace, and this makes the apparatus complicated; and in the case that a porous glass body has a large size, a size of an electric furnace needs to be increased, and this makes the facility constructions difficult.
Furthermore, variation of fluorine introduction amount due to variation of temperature, turbulence of gasflow and the like is generated. If the variation of the fluorine introduction amount is increased, in the case of using as, for example, an optical system member of an exposure device for EUVL, variation of a coefficient of thermal expansion is generated in the plane of the glass, and as a result, there is a problem that the resolution in the exposure decreases.